Infection, Infectious Diseases, and Epidemiology Notes

Symbiotic Relationships Between Microbes and Hosts

  • Symbiosis: "to live together"

Types of Symbiotic Relationships

  • Mutualism: both organisms benefit.

  • Commensalism: one organism benefits, the other is neither harmed nor benefits.

  • Amensalism: one organism is harmed, the other is neither harmed nor benefits.

  • Parasitism: one organism benefits, the other is harmed.

Microbiome of Humans

  • Organisms that colonize the body's surfaces without normally causing disease (normal microbiota/flora, indigenous microbiota).

Types of Microbiota

  • Resident microbiota: remain part of normal microbiota throughout life.

  • Transient microbiota: remain in the body for a short period but cannot persist.

Acquisition of Microbiome

  • Development in the womb: generally free of microorganisms.

  • Microbiome development begins during the birthing process.

  • Most established during the first months of life.

Opportunistic Pathogens

  • Normal microbiota that cause disease under certain circumstances.

Conditions Providing Opportunities for Pathogens

  • Immune suppression.

  • Stressful conditions.

  • Changes in the normal microbiota.

  • Introduction of normal microbiota into unusual sites in the body.

Reservoirs of Infectious Diseases of Humans

  • Most pathogens cannot survive for long outside their host.

  • Reservoirs of infection: sites where pathogens are maintained as a source of infection.

Types of Reservoirs

  • Animal reservoirs.

  • Human carriers.

  • Nonliving reservoirs.

Animal Reservoirs

  • Zoonoses: diseases that naturally spread from animal hosts to humans.
    -Acquired through various routes:
    -Direct contact with animal/its waste.
    -Eating animals.
    -Bloodsucking arthropods.
    -Humans are usually dead-end hosts for zoonotic pathogens.
    -Difficult to eradicate.

Human Carriers

  • Asymptomatic infected individuals can be infective to others.

  • Some individuals eventually develop illness, while others do not.

Nonliving Reservoirs

  • Soil, water, and food can be reservoirs of infection.

  • Contamination often occurs by feces or urine.

Infection

  • Contamination: mere presence of microbes in/on the body.

  • Infection: when organisms evade the body's external defenses, multiply, and become established in the body.

Portals of Entry

  • Sites through which pathogens enter the body.

  • Major pathways: skin, mucous membranes, placenta.

Adhesion in Infection

  • Adhesion: the process by which microorganisms attach themselves to cells.

  • Required to establish colonies successfully within the host.

Adhesion Factors

  • Specialized structures.

  • Attachment molecules (ligands).

  • Attachment proteins: found on viruses and many bacteria, aiding in adhesion.

  • Biofilms: some bacterial pathogens attach to each other to form a biofilm.

Virulence

  • Intensity of disease.

  • Avirulent: will not cause disease.

Manifestations of Disease

  • Symptoms: subjective characteristics of disease only felt by the patient.

  • Signs: objective manifestations of disease observed or measured by others.

  • Syndrome: symptoms/signs that characterize a disease/an abnormal condition.

  • Asymptomatic/subclinical infections: lack symptoms but may still have signs of infection.

Pathogen

  • Giving rise to disease.

Septicemia

  • Pathogens in the blood.

Toxin

  • Harmful compound.

Etiology (Robert Koch)

  • Etiology: study of the cause of disease.

  • Diseases have various causes.

Virulence Factors of Infectious Agents

  • Pathogenicity: ability of a microorganism to cause disease.

  • Virulence: degree of pathogenicity.

Virulence Factors

  • Adhesion factors.

  • Biofilms.

  • Extracellular enzymes.

  • Toxins.

  • Antiphagocytic factors.

Extracellular Enzymes

  • Secreted by the pathogens.

  • Dissolve structural chemicals in the body.

  • Help pathogens maintain infection, invade, enter, and avoid body defenses.

Toxins

  • Chemicals that harm tissues or trigger host immune responses that cause damage.

  • Toxemia: presence of toxins in the bloodstream.

    • Exotoxins.

    • Endotoxins.

Antiphagocytic Factors

  • Allow pathogens to remain in a host for a long time.

    • Bacterial capsule.

    • Antiphagocytic chemicals.

Stages of Infectious Disease

  • Incubation period.

  • Prodromal period.

  • Illness.

  • Decline.

  • Convalescence (recovery).

Portals of Exit

  • Ear (earwax).

  • Broken skin (blood).

  • Skin (flakes).

  • Eyes (tears).

  • Nose (secretions).

  • Mouth (saliva, sputum).

Modes of Infectious Disease Transmission

  • Transmission is from a reservoir or a portal of exit to another host's portal of entry.

Transmission Groups

  • Contact transmission.

  • Vehicle transmission.

  • Vector transmission.

Contact Transmission

  • Direct contact transmission: body contact between hosts.

  • Indirect contact transmission: pathogens are spread from host to host by fomites.

  • Droplet transmission: sneezing, coughing, exhaling.

Vehicle Transmission

  • Airborne transmission: pathogens travel more than 1 meter via an aerosol (sneezing, coughing, etc.).

  • Waterborne transmission: important in the spread of many gastrointestinal diseases; fecal-oral infection.

  • Foodborne transmission: spread in and on foods (inadequately processed, cooked, or refrigerated foods); contaminated with feces and/or urine.

  • Bodily fluid transmission: blood, urine, saliva.

Vector Transmission

  • Biological vectors: biting arthropods.

  • Mechanical vectors: passively transmit on their body to new hosts.

Infectious Disease Classifications

  • Acute: symptoms develop rapidly and runs its course quickly.

  • Chronic: usually mild symptoms that develop slowly last long.

  • Subacute: between acute and chronic.

  • Latent: appears a long time after infection.

  • Communicable: transmitted from host to host.

  • Contagious: easily spread.

Epidemiology

  • Study of where and when diseases occur.

Frequency of Disease

  • Disease occurrence tracked using two measures:

    • Incidence: # of NEW cases in a given area during a given time period.

    • Prevalence: # of TOTAL cases in a given area during a given time period.

Types of Outbreaks

  • Sporadic: outbreak in a small area (confined).

  • Endemic: appears every year (expected).

  • Epidemic: large outbreak in a particular area (one state/country).

  • Pandemic: large outbreak all over (worldwide).

Epidemiological Studies

  • Descriptive epidemiology: careful tabulation of data concerning a disease; record location/time of the cases of disease and collect patient information.

  • Analytical epidemiology: seeks probable cause, mode of transmission, and methods of prevention.

  • Experimental epidemiology: tests a hypothesis concerning the cause of a disease.